Method for cracking oil with accumulation under pressure



Sept. 24, 1935. c. A. HILLMAN M J METHOD FOR CRACKING OIL WITH ACCUMULATION UNDER PRESSURE Filed. April 20, 1951 V Patented Sept. 24, 1935 METHOD FOR CRACKING OIL WITH ACCU- MULATION UNDER PRESSURE Charles A. Hillman, East Orange, N. 3., assignor to Standard Oil Development Company Application April 20, 1931, Serial No. 531,312

2 Claims.

includes improvements in tube and tank cracking equipment in which an accumulator is operated at a pressure greater than that of the fractionating equipment.

The main object of the invention is to make possible a high coil inlet temperature when charging a low boiling feed stock without using high pressure fractionating towers or exchangers. A temperature is maintained in the accumulator, without excessive vaporization of the fresh feed stock, far above that obtainable when the accumulator is held at the pressure of the fractionating equipment. A further object of the invention is to provide means for utilizing the heat of the condensed oil to vaporize the lighter fractions of the feed oil from the charging stock, thereby building up the pressure of the accumulator.

The invention will be fully understood from the following description, read in connection with the accompanying drawing, in which the figure is a diagrammatic side elevation of a cracking plant comprising my improvements.

Referring 'to the drawing, reference numeral I denotes a cracking coil in a furnace setting 2,

and discharging through a pipe 3 into the lower part of an enlarged thermally insulated reaction chamber or soaking drum 4. The cracking equipment itself is not a part of the present invention, and any approved type may be adopted.

The cracked product passes from near the top of the soaking drum 4 through a pipe 5 into a tar separator 6. A pressure release valve 1 is installed in the pipe 5. A drawoif line 8 is connected to the separator 6 and leads to a tar cooler 9. Vapors pass from the separator through a pipe II) to a fractionation means, such as a bubble tower H, preferably entering just above the bottom plate l2 therein. The fractionated product passes from the tower through a pipe l3 to a 45 condensing coil [4 and a receiving drum l5. This latter has a vent pipe IS in which pressure control valve I1 is installed.

Condensate formed in the tower and adapted for recycling, is withdrawn. to the accumulator l8 through line H! by pump 20. When no increase of pressure is desirable in the accumulator, valve 2! controls the flow of condensate through line Ila to the accumulator 'l8, by-passing pump 20. Oil to be cracked is supplied to the accumulator through line 22 and pump 23.

This'line includes a heat exchange coil 2t arranged in the upper part of the tower ll.

The vapors formed inthe accumulator 58 are withdrawn to bubble tower l I by means of line 25. A pressure control valve 26 between accu- 5 mulator l8 and bubble tower ll regulates the pressure. The combined liquid contents of the accumulator are forced by pump 27 through a pipe 28 to the inlet end of the cracking coil I.

The operation of the process will be clear from 10 the following example in which it is assumed that heavy naphtha is to be cracked. The heavy naphtha is pumped through pipe 22 into accumulator l8 until the level therein stands about as shown in the drawing. Pumping of the oil 15 from the accumulator through pipe 28 into the cracking coil l, which is suitably fired, is then begun. During the cracking period the temperature of the oil at the outlet of the coil will be held at about 950 F. and in the drum at about 0 890 F., the pressure in the coil and drum being about 1000 pounds per square inch. The pressure of the accumulator I8 is gradually built up to 2'70 pounds per square inch by means of the vaporization of the feed stock and pumps 23 and 25 20, valve 2| being closed and pressure relief valve 26 being set at 270 pounds per square inch.

The valve 1 will be set to give a pressure of about 1000 pounds on the soaker. A pressure of 100 pounds per square inch is maintained 30 throughout the rest of the system bymeans of valve IT on the receiving drum l5. In some cases it is desirable to maintain a higher pressure in the fractionating equipment, for example 150 pounds per square inch, though this may vary with the characteristics desired in the product.

The temperature of the tar, if any is present in the separator 6, will be about 750 F. when working at 100 pounds pressure, and the vapors emerge from the separator at about the same temperature. At this temperature and 100 pounds pressure, kerosene and heavy naphtha will pass over with the gasoline into the bubble tower. The kerosene and/or any heavier distillate fraction produced is separately collected and returned to the accumulator.

The accumulator may be maintained at a pressure (e. g. 200 to 300 pounds per square inch) substantially greater than in the fractionating equipment, and a temperature of 500 F. to 600 F. The pressure maintained is due to pumps 20, 23, the pressure of which is counterbalanced by the vapor pressure of the accumulator stock, and, as stated, may be 210 pounds per square inch in the particular case herein described. At this temperature and pressure a fraction of the heavy naphtha is vaporized and will pass to the bubble tower by pipe line 25, in order to maintain the desired pressure in the accumulator. The valve 26 maybe set to give the required pressure differential between the accumulator and the bubble tower.

The present invention makes efllcient use of the heating and distilling effect of the hot cracked product. High pressure exchangers are not required nor is the showering of oil through the bubble tower used to obtain preheat, as has heretofore been proposed. Further, the accumulator pressure is maintained by removing a percentage of the lighter fractions of the feed stock boiling below the end point of the desired distillate, thus relieving the cracking coil and allowing it to work in a more satisfactory manner.

It will be understood that the invention is not limited to operation upon the particular stock referred to, and that variations in temperature and pressure are permissible. Numerous changes and alternative arrangements may be made within the scope of the appended claims in which it is my intention to claim all novelty inherent in the invention as broadly as the prior art permits.

I claim:

1. In' the cracking of naphtha the improvement which comprises supplying the naphtha in the preheated condition to an accumulating zone, passing a portion of condensed cracked oil vapors thereinto from a fractionating zone, withdrawing oil from the accumulating zone and passing it through an elongated narrow heating zone where it is heated to cracking temperature, digesting the heated oil in an enlarged digesting zone while maintaining the heating and digesting zones under high pressure, passing the hot fluid from the digesting zone into a tar separating zone, separately removing liquid tar and oil vapors from the separating zone and fractionating the removed oil vapors in said fractionating zone to obtain a condensate and a vapor, condensing the'vapor in a condensing zone, and maintaining a substantially higher pressure in the accumulating zone than in the fractionating zone.

2. The process according to claim 1 in which a pressure of the magnitude of about 270 pounds per square inch is maintained on the accumulating zone and a pressure of about 100 pounds per square inch is maintained on the fractionating zone.

CHARLES A. HILLMAN. 

